No, it's not too early, but there are a few pieces of advice I can offer to better your chances.
Don't come expecting too much. May whale-watchers are often disappointed when they don't see 100 humpbacks and orcas breaching in the harbor... that usually happens in August!
Also, give your boat driver some time. We can all find the same small group of sleeping whales in 2-2.5 hours, but for a truly memorable experience, go for something longer. In May, I always have amazing Tracy Arm or Lighthouse tours. It takes longer than the typical whale-watch trip to get there, but if you've got 6, 8 or 10 hours the wildlife and scenery is amazing that time of year.
Happy Alaska Travels!
Better not for now... signatures get me banned!